Microbial keratitis is a sight-threatening infectious disease associated with significant ocular morbidity. By one estimate, the annual incidence of microbial keratitis is approximately 500,000 patients worldwide and 30,000 patients in the US alone. The ocular surface is generally inhabitable for microbial pathogens because of anatomical barriers that impede microbial colonization, mechanical systems that constantly clear pathogens, and coordinated defense responses that are rapidly mounted to effectively eradicate invading pathogens. Infection occurs when one or several of these mechanisms are compromised and the balance of host-pathogen interactions shifts to favor pathogenic mechanisms. Thus, a better understanding of host-pathogen interactions is a major public health priority, but the bacterial and host mechanisms that contribute to the pathogenesis of microbial keratitis are incompletely understood. The central goal of this application is to define how syndecan- 1, a major cell surface heparin sulfate proteoglycan (HSPG), promotes the pathogenesis of bacterial keratitis. Syndecan-1 is abundantly expressed in the corneal epithelium, but much remains unknown of its functions in the cornea. Syndecan-1 functions as receptors on the cell surface and also as soluble HSPGs in the extracellular environment because its extracellular ectodomain can be shed under certain pathological conditions. Preliminary studies showed that mice deficient in syndecan-1 significantly resist corneal infection by Staphylococcus aureus, suggesting that syndecan-1 is an important host factor that promotes S. aureus keratitis. The underlying mechanisms of how syndecan-1 functions in this manner are not understood, but preliminary data also suggested that S. aureus induces syndecan-1 shedding in corneal epithelial cells and exploits the capacity of syndecan-1 ectodomains to counteract neutrophil-mediated defense mechanisms in a heparin sulfate (HS)-dependent manner. Based on these data, this proposal will examine the overall hypothesis that bacterial subversion of HSPGs is an important virulence mechanism in the pathogenesis of bacterial keratitis in 3 Specific Aims. Aim 1 will define how syndecan-1 shedding is induced in S. aureus keratitis. Aim 2 will elucidate the structural features of syndecan-1 ectodomain HS that promote S. aureus keratitis, and Aim 3 will determine the biological mechanism of how syndecan-1 ectodomains inhibit S. aureus killing by neutrophils. The proposed studies will use both novel and established genetic, biochemical, and cell biological approaches in conjunction with in vivo models of bacterial keratitis. The proposed studies are anticipated to uncover previously unknown biological functions of corneal epithelial HSPGs and to increase our understanding of molecular and cellular processes that are central to the pathogenesis of bacterial keratitis.